"The Department for Children, Schools and Families (DCSF) will not tell us how many students intend to take the course," said a CECA spokesman.
The diploma – of equivalent weight to an A-Level – is due to begin in September.
But it hit two stumbling blocks last week. First exam board Edexcel said teachers were given insufficient training for the diploma. Then CECA criticised the DCSF for not releasing pupil numbers, saying this indicated a low uptake.
"The Government has failed to make it clear to employers what the status of the new diplomas will be or how they can get involved," said CECA director of training Joe Johnson.
The Employer Engagement Group which is developing the diploma hit back by revealing the involvement of 71 major contractors.
CECA say this has not addressed the fundamental question.
"Information the government has put out has been poor – students and parents need to make the decisions for September. We can only conclude that numbers of students enrolled are low," said the spokesman.